1. Field of the Invention
The present invention relates to a cache device, a communication apparatus, and a computer program product.
Priority is claimed on Japanese Patent Application No. 2012-009137, filed Jan. 19, 2012, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
Conventionally, in order to implement a high level of automated operation in plants, factories, and the like, distributed control systems (DCSs), in which on-site equipment (measurement instruments and actuators) known as field devices are connected to device management apparatuses that manage and control the field devices via a communication bus have been implemented. In such a distributed control system, cache devices are often used to alleviate delay in data transfer to the minimum possible, and to improve transmission efficiency.
A cache device used in a distributed control system holds device property information obtained from field devices and parameter property information obtained from a DD (device description) file created using a DDS (data description specification) as cache data in a memory such as a RAM (random-access memory) or the like. In this case, the above-noted device property information is a tag indicating the ID (identifier) or name of a field device, the internal field device calculation time period, and information indicating other characteristics of the field device. The above-noted parameter property information is parameters used in the field device (for example, parameters into which measurement results are stored, or parameters for calculations), such as data length, sign, moniker, and information indicating other characteristics.
Specifically, when there is a parameter read or write request to a field device from a host device (for example, a device management apparatus), the above-noted cache device obtains and holds as cache data the device property information and parameter property information required for the execution of processing in response to the request from the field device and the DD file, and executes processing in response to the request. Then, if the same request is received again, by executing the processing in response to the request using device property information and parameter property information that are held as cache data, the time required for obtaining the device property information from the field device and the time required for analyzing the DD file to obtain the parameter property information are eliminated.
Japanese Unexamined Patent Application, First Publication No. 2003-333121 discloses a field device communication apparatus provided between a host device and a controller that controls a field device, which fully asynchronously communicates with a GUI means of the host device and which communicates with the field device via the controller. Additionally, Japanese Unexamined Patent Application, First Publication No. 2006-91961 discloses a communication interface that has a request cache in which a request from a host application is registered and a response cache in which a response from a field device is stored.
As disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. 2006-91961, requests from a host device and responses from a field device are without exception conventionally all held into memory as cache data. For this reason, even information for which there is no expectation of re-referencing is held as cache data in memory, this leading to unnecessary use of memory capacity. In recent years, communication apparatuses connectable to several thousand field devices have become practical and, with such communication apparatuses, a huge amount of memory to hold cache data is required.
Additionally, because responses and the like from the above-described field devices has conventionally been all held as cache data in memory, the maximum number of field devices to which connection could be made in accordance with a capacity of memory is limited. Although it can be envisioned that if an expansion of memory used for the cache is possible, an increase is possible in the maximum number of field devices, the maximum number of field devices cannot be increased if it is not possible to expand the memory.